Probation officers warned today that scrapping "soft" community sentences would just line privateers' pockets.
Napo assistant general secretary Harry Fletcher urged the House of Lords to veto Justice Secretary Chris Grayling's "chaotic" plans for one-size-fits-all punishments.
Mr Grayling vowed last month to end "soft option" community sentences and make things such as fines, community service and GPS tagging a compulsory part of judges' orders.
Two-thirds of community orders already contain such punishments but the Bill now before the Lords would force judges to slam every offenders with one.
Mr Fletcher told the Morning Star today the Con-Dems had missed the point of community sentences - "and haven't bothered to cost it."
Napo said GPS tagging all offenders would cost "at least" £340m more a year and unpaid work schemes about £80m.
Mr Fletcher warned homeless or otherwise disadvantaged people likely to routinely breach a blanket house arrest.
And throwing them in Britain's overcrowded prisons would only make matters worse.
Mr Fletcher fears that mass tagging could become a pretext for getting rid of probation officers.
"It's all going to be delivered by the private sector - these will not be trained probation officers monitoring them.
"You're looking at six to seven thousand jobs lost because they want to make a profit out of punishment.
"Ultimately it will put public safety at risk."
The Howard League for Penal Reform's campaigns director Andrew Neilson agreed.
"Public confidence in community sentences is based on whether they stop someone committing crime again and don't cost the earth," he said.
"Adding a punitive element to all community sentences is useful for grabbing a few headlines but won't make the public any safer.
"The government's own assessment of this measure suggests reoffending might rise."
The Ministry of Justice said it "totally rejects" the claim.
"Only a system that both properly punishes and effectively rehabilitates offenders will break the cycle of reoffending," it said.
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